1. Field of the Invention
The present invention relates to a collimator lens of a single lens type.
2. Description of the Prior Art
A pickup for laser disk system such as a compact disk player or a digital audio disk player necessitates a collimator lens, by which light generated from a light source of semiconductor laser is condensed and collimated to be, in turn, converged onto a disk through an objective lens arranged next to the collimator lens.
In this field of art, there have been various types of prior art developed including a method of forming a single lens applicable to the collimator lens.
In the Japanese Patent Laid-Open No. 146721/1986, for example, a glass lens forming method is disclosed. In this method, a melted liquid glass is dropped down to the air from a nozzle. During the dropping, a surface of the glass drop is formed into spherical surface by means of the surface tension by itself. The glass drop is received by a mold and then pressed so that the glass gob is made into a glass lens with desired shape. Hereinafter, such a glass lens forming method is referred to as "the liquid glass drop method". The liquid glass drop method has an advantage in that there is no necessity of grinding the lens surface after shaped by the mold.
In the liquid glass drop method, however, the volume of the dropped glass is limited, which makes it impossible to form a lens of an axial distance greater than a limit.
However, a prior art collimator lens of a single lens type such as disclosed in the U.S. Pat. No. 4,613,213, U.S. Pat. No. 4,657,352 and the Japanese Patent Laid-Open No. 132915/1986 each requires an axial distance considerably long in comparison with the radius of curvature. Similarly, the Japanese Patent Laid-Open No. 14611/1987 discloses a microlens for optical fibers, which has a long axial distance. Further, in a viewpoint of correcting a spherical aberration well in a collimator lens, especially a plano-convex lens, whose numerical aperture NA is about 0.25, the axial distance is required to be long enough.
Thus, the prior art has only provided the field of collimator lens with a single lens of a heavy weight because of its long axial distance, which is, on the other hand difficult to be formed by the above-described liquid glass drop method or the like.